A dynamic random access memory (DRAM) cell typically comprises a charge storage capacitor (or cell capacitor) coupled to an access device such as a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET). The MOSFET functions to apply or remove charge on the capacitor, thus affecting a logical state defined by the stored charge. The amount of charge stored on the capacitor is determined by the capacitance C=∈∈o A/d, where ∈ is the dielectric constant of the capacitor dielectric, ∈o is the vacuum permittivity, A is the electrode (or storage node) area, and d is the interelectrode spacing. The conditions of DRAM operation such as operating voltage, leakage rate and refresh rate, will in general mandate that a certain minimum charge be stored by the capacitor.
In the continuing trend to higher memory capacity, the packing density of storage cells must increase, yet each will maintain required capacitance levels. This is a crucial demand of DRAM fabrication technologies if future generations of expanded memory array devices are to be successfully manufactured. Nevertheless, in the trend to higher memory capacity, the packing density of cell capacitors has increased at the expense of available cell area. For example, the area allowed for a single cell in a 64-Mbit DRAM is only about 1.4 μm2. In such limited areas, it is difficult to provide sufficient capacitance using conventional stacked capacitor structures. Yet, design and operational parameters determine the minimum charge required for reliable operation of the memory cell despite decreasing cell area. Several techniques have been developed to increase the total charge capacity of the cell capacitor without significantly affecting the cell area. These include new structures utilizing trench and stacked capacitors, electrodes having textured surface morphology and new capacitor dielectric materials having higher dielectric constants.
As DRAM density has increased (1 MEG and beyond) thin film capacitors, such as stacked capacitors, trenched capacitors, or combinations thereof, have evolved in attempts to meet minimum space requirements. Many of these designs have become elaborate and difficult to fabricate consistently as well as efficiently. Furthermore, the recent generations of DRAMs (4 MEG and 16 MEG, for example) have pushed thin film capacitors technology to the limit of processing capability. Thus, greater attention has been given to the development of thin film dielectric materials that possess a dielectric constant significantly greater (>10×) than the conventional dielectrics used today, such as silicon oxides or nitrides.
Recently, a lot of attention has been paid to Barium Strontium Titanate (BST), Barium Titanate (BT), Strontium Titanate (ST), Lead Zirconate Titanate (PZT) and other high dielectric constant materials as a cell dielectric material of choice of DRAMs. These materials, in particular BST, have a high dielectric constant (>300) and low leakage currents which makes them very attractive for high density memory devices. However, there are some technical difficulties associated with these materials. One problem with incorporating these materials into present day DRAM cell designs is their chemical reactivity with the polycrystalline silicon (polysilicon or “poly”) that conventionally forms the capacitor electrode or a buried electrode contact. Capacitors made by polysilicon-PZT/BST sandwiches undergo chemical and physical degradation with thermal processing. During chemical vapor deposition (CVD) of PZT/BST, oxygen in the ambient tends to oxidize the electrode material. The oxide is undesirable because it has a much lower dielectric constant compared to PZT/BST, and adds in series to the capacitance of the PZT/BST, thus drastically lowering the total capacitance of the capacitor. Therefore, even a thin native oxide layer present on the electrode results in a large degradation in capacitance. Furthermore, even when the electrode proper is made of a noble metal, such as Pt, oxygen will still tend to diffuse through it, contaminating the underlying polycrystalline silicon plug.
Ferroelectric memory devices have been proposed as alternatives to conventional memory devices. Ferroelectric memory devices utilize the spontaneous polarization properties of ferroelectric films to provide data storage elements which offer relatively fast read/write operations compared with conventional storage elements. In addition, using a capacitor having a ferroelectric dielectric as a data storage device for a memory cell can reduce the power consumption of the memory cell and increase operational speed as refresh operations typically are not required to maintain data in the capacitor. Moreover, such a ferroelectric random access memory (FRAM) device may operate from a single power supply voltage.
Generally, two types of FRAM cells are conventionally used: (1) a transistor employing a ferroelectric film as a gate insulation film, and (2) an access transistor connected to a cell capacitor employing a ferroelectric film as a dielectric. Fabrication difficulties associated with the first type of cell include the potential formation of a silicon oxide film by reaction of silicon with oxygen atoms at the interface between the silicon channel region of the transistor and the ferroelectric gate insulation film. In addition, it may be difficult to form a high-quality ferroelectric film due to a lattice constant difference or thermal expansion coefficient difference between the silicon substrate and ferroelectric film.
For these reasons, conventional FRAM devices tend to employ the second structure described above, wherein a cell capacitor uses a ferroelectric dielectric material as a dielectric. Typically, barium strontium titanate (BST) or lead zirconate titanate (PZT) are used for the capacitor dielectric. According to a typical fabrication process, BST or PZT is deposited by a sol-gel process. The annealing temperature of 500 to 650 degrees Celsius used during the heat treatment phase of the sol-gel process may deform a conventional aluminum electrode, or oxidize a tungsten electrode. Therefore, the lower electrode of a ferroelectric capacitor is typically made of platinum because it has a high oxidation resistance and a high melting point.
Platinum is an excellent lower electrode material to use with ferroelectric and high dielectric constant (HDC) dielectric materials. Platinum provides a low energy crystallization surface which catalyzes the formation of perovskite crystals, it maintains its electrical properties at the crystallization temperatures routinely used for sintering ferroelectric and HDC materials, and it is highly compatible with the ferroelectric properties of ferroelectric dielectric materials.
There are disadvantages to using platinum as an electrode, however, which are generally related to semiconductor process integration. Platinum generally allows oxygen to diffuse through it and hence typically allows neighboring materials to oxidize. Platinum also does not normally adhere well to traditional dielectrics such as silicon dioxide, and the high degree of stress placed on the platinum-silicon dioxide bond generated by the crystallization of the ferroelectric or HDC dielectric material peels the platinum off the substrate during processing. It may also rapidly form a silicide at low temperatures, and also may form hillocks which degrade leakage current properties or short out the capacitor. In addition, alpha-particle creation by the radioactive isotope of platinum (Pt-190), which is typically present as a small percentage of the total platinum atoms in a sample, may be detrimental to the electrical functioning of the capacitor.
There is needed, therefore an improved lower electrode for a ferroelectric or high dielectric constant capacitor having the advantages of a platinum electrode while avoiding problems of oxidation and separation from the substrate. A simple method of fabricating an improved lower electrode is also needed.